a) Field of the Invention
This invention relates to a hydraulic drive system for a hydraulic work vehicle such as a hydraulic excavator, and especially to a hydraulic drive system for a hydraulic work vehicle, which is provided with a tilting control unit for performing both control of a delivery flow rate in accordance with a delivery pressure of a variable displacement hydraulic pump and control of torque to be absorbed in the variable displacement hydraulic pump.
b) Description of the Related Art
As prior art of this type, there is known a hydraulic drive system disclosed in Japanese Patent Application Laid-Open (Kokai) No. SHO 2-129401. FIG. 14 is a hydraulic circuit diagram showing the essential construction of a conventional hydraulic drive system for a hydraulic work vehicle as shown in the above laid-open (kokai) publication.
The hydraulic drive system according to this prior art is provided with a motor, namely, engine 1, a variable displacement hydraulic pump 2 operatively driven by the engine 1, a pilot pump 3, a servo piston valve 4 for controlling a displacement of the variable displacement hydraulic pump 2, a control valve 5 for controlling drive of the servo piston valve 4, and an input valve 6 for controlling drive of the control valve 5.
To a first tilting control signal port 8 of the input valve 6, a first tilting control signal line 7 is connected. These first tilting control signal line 7 and first tilting control signal port 8 make up a first tilting control signal guide system which guides, as a first tilting control signal, a delivery pressure of the variable displacement hydraulic pump 2.
To a second tilting control signal port 10 of the input valve 6, a second tilting control signal line 9 is connected. These second tilting control signal line 9 and second tilting control signal port 10 make up a second tilting control signal guide system which guides a second tilting control signal for controlling torque to be absorbed in the variable displacement hydraulic pump 2.
Further, to a third tilting control signal port 12 of the input valve 6, a third tilting control signal line 11 is connected. These third tilting control signal line 11 and third tilting control signal port 12 make up a third tilting control signal guide system which guides, as a third tilting control signal, a delivery pressure of a variable displacement hydraulic pump constituting a hydraulic pressure source of another hydraulic circuit (not shown).
A solenoid-operated valve 13 is arranged between the pilot pump 3 and the above-mentioned second tilting control signal line 9. This solenoid-operated valve 13 is driven by a drive signal outputted from a controller 14 and supplies a pilot pressure, which is produced at the pilot pump 3, as a second tilting control signal to the second tilting control signal line 9. Namely, the pilot pump 3 constitutes a second tilting control signal generating unit.
The above-mentioned servo piston valve 4, control valve 5 and input valve 6 make up a tilting control unit, which controls a displacement of the variable displacement hydraulic pump 2 in such a way that a delivery flow rate of the variable displacement hydraulic pump 2 decreases as a delivery pressure of the variable displacement hydraulic pump 2, that is, a value of the first tilting control signal increases and which also controls the displacement of the variable displacement hydraulic pump 2 in such a way that torque to be absorbed in the variable displacement hydraulic pump 2 decreases as the value of a pilot pressure outputted through the solenoid-operated valve 13, namely, of the second tilting control signal increases.
In the prior art constructed as described above, when the delivery pressure of the variable displacement hydraulic pump 2 becomes high upon driving and operating an unillustrated actuator, in other words, when the value of the first tilting control signal guided via the first tilting control signal line 7 and the first tilting control signal port 8 increases, a piston of the input valve 6 moves in a rightward direction as viewed in FIG. 14 and as a result, the control valve 5 tends to be changed over to a left position against spring force. As a consequence, control is performed so that the servo piston valve 4 moves in the rightward direction as viewed in FIG. 14 and the displacement of the variable displacement hydraulic pump 2 decreases, in other words, the flow rate of pressure oil delivered from the variable displacement hydraulic pump 2 is suppressed.
While no drive signal is outputted from the controller 14 and the solenoid-operated valve 13 is held at a center valve position as illustrated in FIG. 14, the pilot pressure of the pilot pump 3, namely, the second tilting control signal is not guided to the second tilting control signal port 10 through the second tilting control signal line 9, so that the piston of the input valve 6 is not operated by the second tilting control signal and the torque to be absorbed in the variable displacement hydraulic pump 2 is maintained at a predetermined large value.
Now assume that in a state such as that described above, a drive signal is outputted from the controller 14 in response to a change in the rotational speed (which may hereinafter be also called "engine speed") and the solenoid-operated valve 13 is changed over. A pilot pressure of the pilot pump 3 is then guided to the input valve 6 via the solenoid-operated valve 13 and the second tilting control signal port 10, whereby the piston of the input valve 6 moves in the rightward direction as viewed in FIG. 14. As a consequence, the displacement of the variable displacement hydraulic pump 2 is controlled so that torque to be absorbed in the variable displacement hydraulic pump 2 takes a small value not exceeding output torque of the engine 1. This makes it possible to perform engine power control under which work can be continued without stalling of the engine 1 irrespective of changes in the rotational speed of the engine 1.
In the above-described prior art, the solenoid-operated valve 13 remains held at the center valve position if the controller 14 develops a trouble for a certain cause or breaking takes place in a wire through which the drive signal of the controller 14 is guided to the solenoid-operated valve 13. This makes it impossible to guide the second tilting control signal to the second tilting control signal port 10 through the second tilting control signal line 9. Accordingly, the input valve 6 is not operated by the second tilting control signal so that the torque to be absorbed in the variable displacement hydraulic pump 2 is maintained at a predetermined large value.
FIG. 15 illustrates an engine speed/engine output torque characteristic available from the prior art shown in FIG. 14. In FIG. 15, numeral 15 indicates a characteristic curve representing engine output torque, and a straight line indicated by numeral 16 is a characteristic line representing the above-described torque to be absorbed in the variable displacement hydraulic pump 2, said torque having a predetermined large value TA.
As is illustrated in FIG. 15, the torque to be absorbed in the variable displacement hydraulic pump 2 is fixed to one having the large value TA in the prior art if the controller 14 develops a trouble or breaking takes place in the wire connecting the controller 14 and the solenoid-operated valve 13 with each other. When the engine speed changes to a low engine speed NS, the value TN of the engine output torque therefore becomes smaller than the value TA of the torque to be absorbed, resulting in the problem that the engine 1 is stalled. To continue the work performed by the hydraulic work vehicle, a special operation is therefore needed such as increasing the engine speed, leading to a problem that the ease in performing work is significantly lowered.